Organizations that operate in extreme contexts have to develop resilience to ensure the reliability of their operations. While the organizational literature underlines the crucial role of slack when facing unanticipated events, a structural approach to slack says little about the concrete ways in which organizational actors produce and use this slack. Adopting a practice-based perspective during a 14-month ethnographic study in a French Critical Care Unit, we study the slack practices, which consist in gathering, arranging and rearranging resources from both inside and outside the medical unit. This permanent process is captured in a dynamic model connecting situations, their evolutions and slack practices. Our research highlights the importance of situational slack production practices to ensure resilience. We also argue that these micro-practices are constitutive of the context in which actors are evolving. Finally, we discuss why these slack practices, although essential for ensuring resilience, can be endangered by the New Public Management context.